# std::is_rvalue_reference

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< cpp‎ | types

Defined in header `<type_traits>` |  |
template< class T > struct is_rvalue_reference; |  | (since C++11)

`std::is_rvalue_reference` is a UnaryTypeTrait.

Checks whether `T` is an rvalue reference type. Provides the member constant value which is equal to true, if `T` is an rvalue reference type. Otherwise, value is equal to false.

If the program adds specializations for `std::is_rvalue_reference` or `std::is_rvalue_reference_v`, the behavior is undefined.

### Template parameters

T | - | a type to check

### Helper variable template

template< class T > constexpr bool is_rvalue_reference_v = is_rvalue_reference<T>::value; |  | (since C++17)

## Inherited from std::integral_constant

### Member constants

value [static] | true if `T` is an rvalue reference type, false otherwise (public static member constant)

### Member functions

operator bool | converts the object to bool, returns value (public member function)
operator() (C++14) | returns value (public member function)

### Member types

Type | Definition
`value_type` | bool
`type` | std::integral_constant<bool, value>

### Possible implementation

    template<class T> struct is_rvalue_reference : std::false_type {};
    template<class T> struct is_rvalue_reference<T&&> : std::true_type {};

### Example

Run this code

    #include <iostream>
    #include <type_traits>

    class A {};

    static_assert
    (
        std::is_rvalue_reference_v<A> == false and
        std::is_rvalue_reference_v<A&> == false and
        std::is_rvalue_reference_v<A&&> != false and
        std::is_rvalue_reference_v<char> == false and
        std::is_rvalue_reference_v<char&> == false and
        std::is_rvalue_reference_v<char&&> != false
    );

    template <typename T>
    void test(T&& x)
    {
        static_assert(std::is_same_v<T&&, decltype(x)>);
        std::cout << "T\t" << std::is_rvalue_reference<T>::value << '\n';
        std::cout << "T&&\t" << std::is_rvalue_reference<T&&>::value << '\n';
        std::cout << "decltype(x)\t" << std::is_rvalue_reference<decltype(x)>::value << '\n';
    }

    int main()
    {
        std::cout << std::boolalpha;
        std::cout << "A\t" << std::is_rvalue_reference<A>::value << '\n';
        std::cout << "A&\t" << std::is_rvalue_reference<A&>::value << '\n';
        std::cout << "A&&\t" << std::is_rvalue_reference<A&&>::value << '\n';
        std::cout << "char\t" << std::is_rvalue_reference<char>::value << '\n';
        std::cout << "char&\t" << std::is_rvalue_reference<char&>::value << '\n';
        std::cout << "char&&\t" << std::is_rvalue_reference<char&&>::value << '\n';

        std::cout << "\ntest(42)\n";
        test(42);

        std::cout << "\ntest(x)\n";
        int x = 42;
        test(x);
    }

Output:

    A	false
    A&	false
    A&&	true
    char	false
    char&	false
    char&&	true

    test(42)
    T	false
    T&&	true
    decltype(x)	true

    test(x)
    T	false
    T&&	false
    decltype(x)	false

### See also

is_lvalue_reference (C++11) | checks if a type is an lvalue reference (class template)
is_reference (C++11) | checks if a type is either an lvalue reference or rvalue reference (class template)

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